This invention relates to apparatus used in detecting and measuring the symmetrical components in multiphase AC electrical power systems and more particularly to sequence filters which generate an output signal which is proportional to a particular set of symmetrical components.
It is common to represent the sinusoidal currents and voltages of a multiphase AC electrical power system as a set of rotating vectors. Any three vectors, which may be unsymmetrical or unbalanced, can be resolved into a system of three equal vectors and two symmetrical three-vector systems, which are known as the positive and negative phase sequences. The detection of these phase sequences is useful in determining the degree of unbalance of the power system and can be used to indicate the occurrence of a fault in the system. General background information concerning the use of sequence filters in multiphase AC power systems is contained in U.S. Pat. No. 4,210,948, issued July 1, 1980 to Waltz and U.S. Pat. No. 4,366,521, issued Dec. 28, 1982 to Jessee. These patents are hereby incorporated by reference.
In certain alternating current power systems, such as those used on aircraft, variation in frequency may occur. The phase sequence filters used in conjunction with these systems must be substantially unaffected by variations in frequency. The filters of the present invention are selective for frequency and phase sequence, and can be combined with a three-phase self-tuning arrangement to achieve a fast tracking unique transfer characteristic. Using few components and yielding inherently balanced outputs, this filter is advantageous for many control and instrumentation applications.